U.S. patent number 5,024,901 [Application Number 07/346,774] was granted by the patent office on 1991-06-18 for method for depositing highly erosive and abrasive wear resistant composite coating system on a substrate.
This patent grant is currently assigned to Air Products and Chemicals, Inc.. Invention is credited to Duane Dimos, Paul N. Dyer, Diwakar Garg, Carl F. Mueller, Leslie E. Schaffer, Ernest L. Wrecsics.
United States Patent |
5,024,901 |
Garg , et al. |
* June 18, 1991 |
Method for depositing highly erosive and abrasive wear resistant
composite coating system on a substrate
Abstract
The method for producing the disclosed material comprises
chemical vapor depositing on the substrate a substantially
columnar, intermediate layer of tungsten and chemical vapor
depositing on the intermediate layer a non-columnar, substantially
lamellar outer layer of a mixture of tungsten and tungsten carbide.
The tungsten carbide comprises W.sub.2 C, W.sub.3 C, or a mixture
of both wherein the ratio of the thickness of the tungsten
intermediate layer to the thickness of the outer layer is at least:
(a) 0.35 in the case of tungsten plus W.sub.2 C in the outer layer,
(b) 0.6 in the case of a mixture of tungsten and W.sub.3 C in the
outer layer and (c) 0.35 in the case of mixtures of tungsten and
W.sub.2 C and W.sub.3 C in the outer layer. The chemical vapor
deposition steps are carried out at pressures within the range of 1
Torr to 1,000 Torr and temperatures within the range of about
300.degree. to about 650.degree. C.
Inventors: |
Garg; Diwakar (Macungie,
PA), Dyer; Paul N. (Allentown, PA), Schaffer; Leslie
E. (Macungie, PA), Wrecsics; Ernest L. (Bethlehem,
PA), Dimos; Duane (Upper Mount Clair, NJ), Mueller; Carl
F. (Easton, PA) |
Assignee: |
Air Products and Chemicals,
Inc. (Allentown, PA)
|
[*] Notice: |
The portion of the term of this patent
subsequent to April 9, 2008 has been disclaimed. |
Family
ID: |
26850807 |
Appl.
No.: |
07/346,774 |
Filed: |
May 3, 1989 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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153738 |
Feb 8, 1988 |
4855188 |
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Current U.S.
Class: |
428/627;
427/249.18; 427/250; 427/252; 427/253; 427/404; 427/405; 427/419.7;
428/628; 428/665; 428/680 |
Current CPC
Class: |
C23C
16/14 (20130101); C23C 16/30 (20130101); C23C
28/00 (20130101); Y10T 428/12583 (20150115); Y10T
428/12944 (20150115); Y10T 428/1284 (20150115); Y10T
428/12576 (20150115) |
Current International
Class: |
C23C
16/06 (20060101); C23C 16/14 (20060101); C23C
16/30 (20060101); C23C 28/00 (20060101); B32B
015/04 (); C23C 016/32 (); C23C 016/14 (); C23C
016/46 () |
Field of
Search: |
;427/249,255.2,255.7,250,252,253,404,405,419.7
;428/627,628,665,680 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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690007 |
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Jul 1964 |
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CA |
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61-157681 |
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Jul 1986 |
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JP |
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62-290871 |
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Dec 1987 |
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JP |
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Primary Examiner: Morgenstern; Norman
Assistant Examiner: Bueker; Margaret
Attorney, Agent or Firm: Dannells, Jr.; Richard A. Simmons;
James C. Marsh; William F.
Parent Case Text
This is a division of application Ser. No. 07/153,738, filed Feb.
8, 1988, now U.S. Pat. No. 4,855,188.
Claims
What is claimed is:
1. A method for producing a highly erosion and abrasion wear
resistant composite coating system on a substrate selected from the
group consisting of ferrous metals and alloys, non-ferrous metals
and alloys, graphite, carbides and ceramics, comprising, chemical
vapor depositing on the substrate a substantially columnar,
intermediate layer of tungsten of sufficient thickness to confer
substantial wear resistant characteristics on said coating system
using a gaseous mixture of tungsten halide and hydrogen at
pressures within the range of about 1 Torr. to about 1,000 Torr and
a temperature of about 300.degree. to about 650.degree. C., and
chemical vapor depositing on said intermediate layer a
non-columnar, substantially lamellar outer layer of a mixture of
tungsten and tungsten carbide using a gaseous mixture of tungsten
halide, hydrogen and an oxygen-containing hydrocarbon at pressures
within the range of 1 Torr to about 1,000 Torr and temperatures
within the range of about 300.degree. to about 650.degree. C., with
said tungsten carbide comprising W.sub.2 C, W.sub.3 C or a mixture
of both, wherein the ratio of the thickness of the tungsten
intermediate layer to the thickness of the outer layer is at least
0.35 in the case of tungsten plus W.sub.2 C in the outer layer,
0.60 in the case of tungsten plus W.sub.3 C in the outer layer and,
0.35 in the case of mixtures of tungsten and W.sub.2 C and W.sub.3
C in the outer layer.
2. A method according to claim 1 wherein said intermediate layer is
at least about two microns thick.
3. A method according to claim 1 wherein a primary layer comprised
nickel is deposited on the substrate prior to deposition of the
intermediate layer.
4. The highly erosion and abrasion wear resistant composite coating
system on a substrate prepared in accordance with the method of
claim 1 for use as compressor blades for gas turbines and jet
engines.
Description
BACKGROUND OF THE INVENTION
A. Field of the Invention
The invention relates to highly erosive and abrasive wear resistant
composite coating. More particularly, the invention relates to an
improved highly erosive and abrasive wear resistant coating
comprising a composite coating system of an intermediate layer of
substantially pure tungsten and an outer two phase layer of a
mixture of tungsten and tungsten carbide.
B. Background Art
High hardness materials are widely used as coatings on various type
of mechanical components and cutting tools. Such coatings impart
erosion and abrasion wear resistance and thus increase the erosive
and abrasive wear life of objects that have been coated. The high
hardness materials can also be used to produce free standing
objects which are erosive and abrasive wear resistant.
Chemical vapor deposition processes can be used to produce highly
erosive and abrasive wear resistant hard coatings and free standing
objects. In a typical chemical vapor deposition (CVD) process, the
substrate to be coated is heated in a suitable chamber and then a
gaseous reactant mixture is introduced into the chamber. The
gaseous reactant mixture reacts at the surface of the substrate to
form a coherent and adherent layer of the desired coating. By
varying the gaseous reactant mixture and the CVD process
parameters, various types of deposited coatings can be
produced.
In U.S. patent application Ser. No. 092,809, filed 3 Sept. 1987,
now U.S. Pat. No. 4,874,642, issued 17 Oct. 1989, extremely hard,
fine grained, non-columnar, substantially lamellar tungsten/carbon
alloys are described which are produced by chemical vapor
deposition. The described alloys consist primarily of a mixture of
a substantially pure tungsten phase and at least one carbide phase
wherein the carbide phase consists of W.sub.2 C or W.sub.3 C or a
mixture of W.sub.2 C and W.sub.3 C. The disclosed tungsten/carbon
alloys are free of columnar grains and consist essentially of
extremely fine, equiaxial crystals.
It has been found that the tungsten/carbon alloys such as those
described in the aforementioned U.S. patent application, when
deposited upon certain types of substrates, exhibit a very fine
micro-crack system throughout the deposit. On many types of
substrates and under many types of erosive and abrasive wear
conditions, preferential attack occurs at the cracks, resulting in
poor erosion and abrasion wear resistance for such coatings.
The use of an intermediate layer of substantially pure tungsten
followed by a tungsten carbide coating is described in the prior
art. For example, U.S. Pat. No. 3,389,977 discloses a method of
depositing substantially pure tungsten carbide in the form W.sub.2
C wherein the adherence of W.sub.2 C to a steel substrate is
improved by first cleaning the surface and then depositing a thin
film of tungsten. The thin film of tungsten is deposited at or
above 600.degree. C., making the use of the deposition process
unsuitable for providing erosive and abrasive wear resistance
coating on various carbon steels, stainless steels, nickel and
titanium alloys without severely degrading their mechanical
properties. Additionally, pure W.sub.2 C deposited according to
this patent consists of columnar grains as opposed to non-columnar
grains described in the present patent application. Other instances
of the use of very thin tungsten intermediate layers, often as a
diffusion layer, are reported in other prior art in order to
improve adhesion of tungsten carbide on a substrate. However, there
is no report in the prior art of the effect of a tungsten
interlayer on coating properties of the final coating system nor
has the effect of such a tungsten intermediate layer on the
reduction or elimination of cracks in the outer coating been
reported.
SUMMARY OF THE INVENTION
Very generally, the highly erosive and abrasive wear resistant
composite coating system of the invention comprises an intermediate
layer of tungsten and an outer layer of tungsten/carbon alloy
coating. The intermediate layer of tungsten is of sufficient
thickness to confer substantial erosive and abrasive wear
resistance characteristics to the composite coating system. The
outer tungsten/carbon alloy layer is comprised of a mixture of
tungsten and tungsten carbide, with the tungsten carbide phase
comprising of W.sub.2 C, W.sub.3 C or a mixture of both. The ratio
of the thickness of the intermediate or inner layer to the
thickness of the outer layer is at least above 0.3 in the cases of
W+W.sub.3 C, W+W.sub.2 C+W.sub.3 C and W+W.sub.2 C coatings.
Preferably the ratio of the thickness of the inner layer to the
thickness of the outer layer to get optimum erosion and abrasion
wear performance is at least 0.35 in the case of mixtures of
tungsten and W.sub.2 C in the outer layer, 0.60 in the case of
mixtures of tungsten and W.sub.3 C in the outer layer and, 0.35 in
the case of mixtures of tungsten and W.sub.2 C and W.sub.3 C in the
outer layer.
The tungsten/carbon alloys or coatings consisting of a mixture of
tungsten and tungsten carbide, with the tungsten carbide phase
comprising W.sub.2 C, W.sub.3 C or mixtures of both are defined
herein as tungsten/tungsten carbide to simplify the
description.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is the photomicrograph at 1,000 magnification of the
tungsten coating on AM-350 stainless steel showing a rough surface
finish but the absence of cracks.
FIG. 2 is the cross-sectional view at 3,000 magnification of the
tungsten coating on AM-350 stainless steel showing columnar growth
structure.
FIG. 3 is the schematic of the unetched cross-sectional view at
2,000 magnification of the composite coating system on AM-350
stainless steel substrate constructed in accordance with the
invention.
FIG. 4 is the schematic of the cross-sectional view at 2,000
magnification of the composite coating system on AM-350 stainless
steel etched with the murakami solution constructed in accordance
with the invention.
FIG. 5 is the photomicrograph of the W+W.sub.3 C coating without
the tungsten interlayer at 1,000 magnification on AM-350 stainless
steel showing a network of interconnected cracks.
FIG. 6 is the photomicrograph of the W+W.sub.3 C coating with the
tungsten interlayer at 1,000 magnification on AM-350 stainless
steel showing a few interconnected cracks.
FIG. 7 is the photomicrograph of the W+W.sub.3 C coating with the
tungsten interlayer at 1,000 magnification on AM-350 stainless
steel showing the absence of cracks.
FIG. 8 is the photomicrograph of the surface of W+W.sub.3 C coating
without the tungsten interlayer on AM-350 stainless steel at 100
magnification, scratched with a diamond stylus and showing
significant loss of the coating in the 30-40 Newton load range.
FIG. 9 is the photomicrograph of the surface of W+W.sub.3 C coating
with the tungsten interlayer on AM-350 stainless steel at 100
magnification, scratched with a diamond stylus and showing
significantly reduced loss of the coating in the 30-40 Newton load
range.
FIG. 10 is the photomicrograph of the W+W.sub.2 C+W.sub.3 C coating
without the tungsten interlayer on AM-350 stainless steel at 1,000
magnification showing a network of cracks.
FIG. 11 is the photomicrograph of the W+W.sub.2 C+W.sub.3 C coating
with the tungsten interlayer on AM-350 stainless steel at 1,000
magnification showing the absence of cracks.
FIG. 12 is the photomicrograph of the W+W.sub.2 C coating with the
tungsten interlayer on AM-350 stainless steel at 1,000
magnification showing the presence of a fine crack.
FIG. 13 is the photomicrograph of the W+W.sub.2 C coating with the
tungsten interlayer on AM-350 stainless steel at 1,000
magnification showing the absence of cracks.
FIG. 14 is the graph illustrating the relationship between the
erosion rate and the ratio of the tungsten to the tungsten/carbon
alloy coating thicknesses on AM-350 stainless steel, and
FIG. 15 is the graph illustrating the relationship between the
tungsten/carbon alloy coating thickness and the ratio of the
thicknesses of the tungsten layer to the tungsten/carbon alloy
layer on AM-350 stainless steel.
DETAILED DESCRIPTION OF THE INVENTION
In a preferred form of the invention, the intermediate layer of
tungsten is produced by chemical vapor deposition under
sub-atmospheric to slightly atmospheric pressure, i.e., within the
range of about 1 Torr. to about 1,000 Torr., at a temperature of
about 300.degree. to about 650.degree. C., using a mixture of
tungsten halide such as WF.sub.6, hydrogen, and an inert gas such
as argon. The intermediate layer is of a sufficient thickness to
confer substantial erosive and abrasive wear resistance
characteristics on the composite system. The specific thickness
necessary to do this for various composite coating systems will
become readily apparent to those skilled in the art from the
teaching of this specification, particularly in connection with the
examples set forth below. The intermediate layer of tungsten should
be at least about two microns thick and, for most systems, will be
greater than about three microns.
Following deposition of the intermediate layer of substantially
pure tungsten, an outer layer of tungsten/tungsten carbide is
deposited under sub-atmospheric to slightly atmospheric pressure,
i.e., within the range of about 1 Torr. to about 1,000 Torr., at
temperatures in the range of about 300.degree. to about 650.degree.
C. This outer layer may be either a two phase layer comprising
tungsten and W.sub.2 C or tungsten and W.sub.3 C. Alternatively,
this outer layer may be a three phase layer comprising tungsten,
W.sub.2 C and W.sub.3 C. The relative proportions of the tungsten,
W.sub.2 C, and W.sub.3 C may be selected in accordance with the
particular properties desired in the final composite coating
system. In order to achieve such proportions, the tungsten/tungsten
carbide deposit is applied utilizing a flow of tungsten halide such
as WF.sub.6, argon, hydrogen and an oxygen containing hydrocarbon
such as dimethylether (DME). By controlling the temperature, W/C
atomic ratio in the feed gas and the ratio of hydrogen to WF.sub.6
during the deposition reaction, the particular desired chemical
composition of the tungsten/tungsten carbide layer may be obtained.
Details of the foregoing described process may be found in
applicant's U.S. patent application Ser. No. 92,809 now U.S. Pat.
No. 4,874,642, issued 17 Oct. 1989.
In accordance with the present invention, it has been found that
the ratio of the thickness of the inner tungsten layer to the
thickness of the outer multi-phase tungsten/tungsten carbide layer
has a profound affect on the erosive and abrasive wear resistance
properties of the resulting composite coating system. Although the
reasons for this improvement in the erosive and abrasive wear
resistance are yet not fully understood, it is believed that the
use of the tungsten intermediate layer together with the specified
ratios set forth below refine the micro-crack structure in the
outer deposit so that, even though preferential attack along the
crack system occurs, the rate of attack is greatly attenuated.
Moreover, as set out below, under certain conditions a crack free
outer layer may be achieved.
More specifically, the ratio of the thickness of the inner tungsten
layer to the thickness of the outer tungsten/tungsten carbide
layer, in accordance with the composite coating system of the
invention, is at least above 0.30 with the W+W.sub.3 C, W+W.sub.2
C+W.sub.3 C and W+W.sub.2 C coatings. More specifically, to obtain
optimum erosion and abrasion wear performance the thickness ratio
is at least: 0.35 in the case of mixtures of tungsten and W.sub.2 C
in the outer layer, 0.60 in the case of mixtures of tungsten and
W.sub.3 C in the outer layer, and 0.35 in the case of mixtures of
tungsten and W.sub.2 C and W.sub.3 C in the outer layer. Using
these minimum ratios, superior erosive and abrasive wear resistance
can be achieved. Moreover, by using the ratios set forth above,
under certain conditions, completely crack free outer layers may be
achieved.
The inner tungsten layer is substantially columnar in its grain
structure with the longer dimension of the grains extending
generally perpendicular to the substrate surface. On the other
hand, the grain structure of the tungsten/tungsten carbide outer
layer is very fine-grained, equiaxed, non-columnar, and
substantially lamellar typically of the order of one micron or less
in size. Such structures may be readily achieved using the method
described in the aforementioned U.S. patent application.
The present composite coating system of the invention can be
deposited on a number of ferrous metals and alloys such as cast
irons, carbon steels, stainless steels and high speed steels,
non-ferrous metals and alloys such as copper, nickel, platinum,
rhodium, titanium, aluminum, silver, gold, niobium, molybdenum,
cobalt, tungsten, rhenium, copper alloys and nickel alloys such as
inconel and monel, titanium alloys such as Ti/Al/V, Ti/Al/Sn,
Ti/Al/Mo/V, Ti/Al/Sn/Zr/Mo, Ti/Al/V/Cr, Ti/Mo/V/Fe/Al,
Ti/Al/V/Cr/Mo/Z and Ti/Al/V/Sn alloys, non-metals such as graphite,
carbides such as cemented carbide, and ceramics such as silicon
carbide, silicon nitride, alumina, etc. In depositing the composite
coating system on reactive substrate materials, such as cast irons,
carbon steels, stainless steels, high speed steels, and nickel and
monel alloys, it is preferred to coat the substrate first with a
more noble material such as nickel, cobalt, copper, silver, gold,
platinum, palladium or iridium, by electrochemical or electroless
techniques or by a physical vapor deposition technique such as
sputtering. In depositing the composite coating system on reactive
titanium or titanium alloys, it is also preferred to coat the
substrate first with a more noble material described above by
electroless technique or by physical vapor deposition technique
such as sputtering. It is also preferred to coat the substrate
first with a thin layer of a more noble material described above by
electroless technique followed by another thin layer of a more
noble material by electrochemical or physical vapor deposition
technique. It is also preferred to clean the surface of the
titanium or titanium alloy substrate first and heat treat the noble
metal deposit after depositing on the substrate. The deposition of
noble metal and subsequent heat treatment steps on titanium or
titanium alloys are described in detail in U.S. patent application
Ser. No. 139,891, filed 31 Dec. 1987 now U.S. Pat. No. 4,902,535,
issued 20 Feb. 1990. It is also preferred that upper limit of the
deposition temperature be about 525.degree. C. when depositing the
present composite coating system on titanium and titanium alloys to
minimize the degradation of the mechanical properties. No
deposition of the noble material, however, is required for coating
non-reactive materials such as copper, nickel, cobalt, silver,
gold, platinum, rhodium, niobium, molybdenum, tungsten, rhenium,
graphite, carbides and ceramics. Free standing parts of the
composite coating of the present invention can be made by
depositing it on the substrates such as copper, nickel, cobalt,
silver, gold, molybdenum, rhenium, and graphite and then removing
these substrates by grinding and chemical or electrochemical
etching.
To further illustrate the present invention, the following data are
set forth with respect to a number of coating systems.
A number of ferrous and non-ferrous metals and alloys were used as
substrates in the following coating experiments. Specimens of
AM-350 and SS-422 stainless steels Inconel and IN-718, a well known
nickel alloy, were electroplated with 2 to 5 .mu.m thick nickel
before coating them with tungsten and tungsten/tungsten carbide to
protect them from the attack of hot and corrosive HF acid gas
produced as a by-product in the CVD process. Specimens made of
titanium alloy such as Ti/6Al/4V, on the other hand, were plated
with 2 to 5 .mu.m thick nickel using an electroless technique
described in detail in U.S. patent application Ser. No. 139,891,
filed 31 Dec. 1987 now U.S. Pat. No. 4,902,535, issued 20 Feb. 1990
before coating them with tungsten and tungsten/tungsten
carbide.
EROSION TEST PROCEDURE
The erosion resistance performance of the uncoated and the coated
specimens was determined using a miniature sandblast unit; the test
parameters are summarized in Table 1. The fine alumina powder,
which was used as the erosive material, provided a very harsh
erosion environment, as compared to sand erosion; consequently, an
accelerated test procedure could be used. Two essentially
equivalent techniques were used to evaluate the erosion resistance
of the specimens. The first technique involved measuring the time
it took for the erosive material to penetrate the tungsten/tungsten
carbide coating, which is called the breakthrough time. Penetration
of the tungsten/tungsten carbide coating was visibly apparent as a
color change at the center of the erosion pit; the fact that this
color change corresponded to the initial penetration of the
tungsten/tungsten carbide coating was confirmed by microscopic
examination of erosion pits in cross section. The second technique
involved measuring the weight of a specimen that was lost during an
erosion test for a given time; this time was always less than the
breakthrough time so that only the weight loss of the coating was
measured. The erosion rate was then calculated as the time required
to penetrate the coating on a per mil basis or as the average
weight loss for a 30 second erosion test, respectively.
EXAMPLES
EXAMPLE 1
Uncoated specimens of AM-350 stainless steel and Ti/6Al/4V were
eroded with alumina for two minutes (120 seconds). The depth of the
crater was measured to calculate the erosion rate. The calculated
erosion rate was 60 and 50 seconds/mil for AM-350 and Ti/6Al/4V
specimens, respectively.
EXAMPLE FOR TUNGSTEN COATING
EXAMPLE 2
A number of AM-350, Ti/6Al/4V and IN-718 specimens (0.095
inch.times.1 inch.times.2 inch) were placed in an inductively
heated graphite furnace inside a gas-tight quartz envelope. The
specimens were heated to 460.degree. C. in the presence of flowing
argon gas and at the temperature a gaseous mixture of 300 cc/min of
WF.sub.6, 3,000 cc/min of hydrogen, and 4,000 cc/min of argon was
passed into the furnace over the specimens. The total pressure
within the system was maintained at 40 Torr. The deposition was
conducted for 15 minutes; thereafter, the flow of the reactive
gases was stopped and the specimens were cooled.
The specimens were found to be coated with a dull, adherent,
coherent, and uniform coating. The coating thickness on stainless
steel specimens was .about.12 .mu.m on each side (see Table 2). The
coating had a rough surface finish and was free of cracks, as shown
in the FIG. 1. The coating consisted of columnar growth structure,
as shown in the FIG. 2. X-ray diffraction analysis showed the
presence of only tungsten in the coating. It had a hardness of 455
Vickers, as shown in Table 3. The coating showed very poor erosion
performance; time required to penetrate the coating was only 3
seconds, resulting in a erosion rate of 6 seconds/mil. This,
therefore, indicated that CVD tungsten could not be used to provide
erosion protection.
TABLE 1 ______________________________________ Erosion Test
Procedure ______________________________________ Nozzle Diameter
0.018 inch Stand off Distance 0.6 inch Erosion Media Fine Alumina
Powder (50 .mu.m Average Particle Size) Supply Pressure 32 psig
Flow Rate of Erosion 1.6 g/min Media Erosion Test Standard
Breakthrough Time and Weight Loss
______________________________________
TABLE 2
__________________________________________________________________________
Example 2 Example 3 Example 4A Example 4B Example
__________________________________________________________________________
4C Experiment No. 50 36 37 33 42 Substrate AM-350 AM-350 AM-350
Ti/6A1/4V AM-350 AM-350 Ti-6A1/4V Temperature, .degree.C. 460 460
460 465 460 Pressure, Torr 40 40 40 40 40 W Coating Conditions
H.sub.2, SCCM 3,000 -- 3,000 3,000 3,000 Ar, SCCM 4,000 -- -- -- --
WF.sub.6, SCCM 300 -- 300 300 150 Time, Min. 15 -- 5 5 30
Tungsten/Tungsten Carbide Conditions H.sub.2, SCCM -- 3,000 3,000
3,000 3,000 Ar, SCCM -- -- -- -- -- WF.sub.6, SCCM -- 300 300 300
300 DME, SCCM -- 40 40 40 40 Time, Min. -- 40 55 35 20 Coating
Thickness, .mu.m Tungsten 12.0 -- 3.0 2.0 3.0 5.0 3.0
Tungsten/Tungsten Carbide -- 22.0 27.0 28.0 25.0 13.0 14.0 Tungsten
Coating Thickness Tungsten/Tungsten Carbide -- 0.0 0.11 0.07 0.12
0.38 0.21 Thickness Surface Topography Rough, Smooth, Smooth, Many
Fine Smooth, Smooth, Many Fine No Cracks Many Fine Interconnected
Many Fine Interconnected Interconnected Cracks Interconnected
Cracks Cracks Cracks
__________________________________________________________________________
Example 4D Example 4E Example 4F Example 4G Example
__________________________________________________________________________
4H Experiment No. 38 107 104 106 164 Substrate AM-350 AM-350 AM-350
Ti/6A1/4V AM-350 AM-350 Ti/6A1/4V IN-718 Temperature, .degree.C.
460 460 460 460 460 Pressure, Torr 40 40 40 40 40 W Coating
Conditions H.sub.2, SCCM 3,000 3,000 3,000 3,000 3,000 Ar, SCCM --
4,500 4,500 4,500 4,500 WF.sub.6, SCCM 300 300 300 300 300 Time,
Min. 10 15 15 15 25 Tungsten/Tungsten Carbide Conditions H.sub.2,
SCCM 3,000 3,000 3,000 3,000 3,000 Ar, SCCM -- 300 300 300 300
WF.sub.6, SCCM 300 300 300 300 300 DME, SCCM 40 30 50 40 40 Time,
Min. 30 30 30 30 25 Coating Thickness, .mu.m Tungsten 8.0 8.5 9.0
8.0 9.0 16.9 17.1 16.9 Tungsten/Tungsten Carbide 16.0 12.5 11.5
11.0 11.0 14.5 13.3 11.7 Tungsten Coating Thickness
Tungsten/Tungsten Carbide 0.50 0.68 0.78 0.73 0.81 1.17 1.29 1.44
Thickness Surface Topography Smooth, Smooth, A Few Smooth, A Few
Fine Smooth, A Smooth, A Few Fine Several Fine Fine Inter-
Interconnected Few Fine Interconnected Inter- Connected Cracks
Interconnected Cracks connected Cracks Cracks Cracks
__________________________________________________________________________
Example 4I Example 4J Example 4K
__________________________________________________________________________
Experiment No. 177 167 160 Substrate AM-350 Ti/6A1/4V IN-718 AM-350
Ti/6A1/4V IN-718 AM-350 Ti/6A1/4V IN-718 Temperature, .degree.C.
460 460 460 Pressure, Torr 40 40 40 W Coating Conditions H.sub.2,
SCCM 2,500 3,000 2,500 Ar, SCCM 6,000 4,500 6,000 WF.sub.6, SCCM
250 300 250 Time, Min. 25 25 25 Tungsten/Tungsten Carbide
Conditions H.sub.2, SCCM 3,000 3,000 3,000 Ar, SCCM 2,000 300 2,000
WF.sub.6, SCCM 300 300 300 DME, SCCM 40 40 40 Time, Min. 35 20 15
Coating Thickness, .mu.m Tungsten 11.0 10.5 10.3 15.5 15.7 15.0
13.2 12.5 12.5 Tungsten/Tungsten Carbide 7.7 7.8 7.7 8.7 9.0 9.0
4.4 3.4 4.0 Tungsten Coating Thickness Tungsten/Tungsten Carbide
1.42 1.35 1.34 1.78 1.74 1.67 3.00 3.67 3.12 Thickness Surface
Topography Smooth, A Few Fine Smooth, A Few Fine Smooth, No Cracks
Interconnected Interconnected Cracks Cracks
__________________________________________________________________________
EXAMPLE FOR TUNGSTEN/TUNGSTEN CARBIDE (W+W.sub.3 C) COATING
EXAMPLE 3
In this example, several specimens of AM-350, Ti/6Al/4V and IN-718
were coated simultaneously in a single run. The specimens were
heated to a temperature of about 460.degree. C. in the presence of
flowing argon and at the reaction temperature a gaseous mixture of
300 cc/min WF.sub.6, 3,000 cc/min of hydrogen and 40 cc/min of DME
was passed into the furnace over the specimens. The total pressure
was maintained at 40 Torr, as shown in Table 2. The deposition was
conducted for 40 minutes.
All the specimens were coated with a bright, smooth, adherent,
coherent and uniform coating. The coating thickness on stainless
steel specimens was .about.22 .mu.m. The coating consisted of a
mixture of W and W.sub.3 C phases, as determined by x-ray
diffraction. It was free of columnar grains. The coating had a
smooth surface finish. However, the surface of the coating was
heavily cracked, as shown in the FIG. 5. The coating had a hardness
of 1788 Vickers, as shown in Table 3. The coating showed poor
erosion resistance; the breakthrough time and erosion rate were 36
seconds and 42 seconds/mil, respectively. The weight loss during
erosion test was 0.00036 g in 30 seconds. Extensive chipping of the
coating was observed during the erosion test. Poor erosion
resistance of the coating was probably due to presence of a network
of cracks in the coating.
EXAMPLES FOR TUNGSTEN FOLLOWED BY TUNGSTEN/TUNGSTEN CARBIDE
(W+W.sub.3 C) COATING
EXAMPLE 4A
In this example, a two step coating process was used. Several
AM-350, Ti/6Al/4V and IN-718 specimens were heated to a temperature
of about 460.degree. C. in the presence of flowing argon and at the
reaction temperature a gaseous mixture of 300 cc/min WF.sub.6, and
3,000 cc/min of hydrogen was passed into the furnace over the
specimens for 5 minutes to coat them with tungsten. After coating
specimens with tungsten for 5 minutes, a gaseous mixture of 300
cc/min WF.sub.6, 3,000 cc/min hydrogen and 40 cc/min of DME was
passed into the furnace for 55 minutes to provide tungsten/tungsten
carbide coating. A total pressure was maintained at 40 Torr during
tungsten as well as tungsten/tungsten carbide coating steps (see
Table 2).
The stainless steel and Ti/6Al/4V specimens were coated with 2-3
.mu.m thick tungsten followed by 27-28 .mu.m thick
tungsten/tungsten carbide coating as shown in Table 2. The
tungsten/tungsten carbide top coat consisted of a mixture of W and
W.sub.3 C phases as shown in Table 3. The hardness values of the
coating on AM-350 and Ti/6Al/4V are summarized in Table 3. The
coating on AM-350 and Ti/6Al/4V showed the presence of a network of
cracks. Erosion resistance of the coating was extremely poor, as
shown in Table 3. Additionally, extensive chipping of the coating
was observed during the erosion test. Poor erosion resistance of
the coating was probably due to extensive cracking of the
coating.
This example described that providing a very thin interlayer of
tungsten did not help in improving erosion resistance of the
overall composite coating.
EXAMPLE 4B
The CVD run described in Example 4A was repeated to provide
tungsten followed by tungsten/tungsten carbide coatings. The
reaction conditions used in tungsten and tungsten/tungsten carbide
coating steps are summarized in Table 2.
The stainless steel specimens were coated with 3 .mu.m tungsten
followed by 25 .mu.m of tungsten/tungsten carbide. The top coat
consisted of a mixture of W and W.sub.3 C phases. The coating
showed the presence of a network of cracks. Erosion resistance of
the coating improved slightly, but it was still extremely poor as
shown in Table 3. Additionally, extensive chipping of the coating
was observed during the erosion test. Poor erosion resistance was
due to the presence of a network of cracks in the coating.
This example described that increasing the ratio of the thickness
of the tungsten to the tungsten/tungsten carbide layer helped in
improving the erosion resistance of the composite coating.
TABLE 3
__________________________________________________________________________
Example 2 Example 3 Example 4A Example 4B Example
__________________________________________________________________________
4C Experiment No. 50 36 37 33 42 Substrate AM-350 AM-350 AM-350
Ti/6A1/4V AM-350 AM-350 Ti/6A1/4V Coating Composition W W + W.sub.3
C W + W.sub.3 C W + W.sub.3 C W + W.sub.3 C Coating Hardness,
Kg/mm.sup.2 Tungsten Layer 455 .+-. 50 -- -- -- -- --
Tungsten/Tungsten Carbide -- 1788 .+-. 130 2276 .+-. 103 2150 .+-.
128 2333 .+-. 165 -- -- Layer Erosion Resistance Breakthrough Time,
Secs 3 36 42 34 68 43 45 Calculated Erosion Rate, 6 42 40 31 69 84
82 secs/mil Weight Loss in 30 seconds, g -- 0.00036 -- -- 0.00040
-- --
__________________________________________________________________________
Example 4D Example 4E Example 4F Example 4G Example
__________________________________________________________________________
4H Experiment No. 38 107 104 106 164 Substrate AM-350 AM-350 AM-350
Ti/6A1/4V AM-350 AM-350 Ti/6A1/4V IN-718 Coating Composition W +
W.sub.3 C W + W.sub.3 C W + W.sub.3 C W + W.sub.3 C W + W.sub.3 C
Coating Hardness, Kg/mm.sup.2 Tungsten Layer -- -- -- -- -- -- --
-- Tungsten/Tungsten Carbide 2164 .+-. 264 2395 .+-. 15 2361 .+-.
103 2328 .+-. 203 2470 .+-. 53 2424 2175 2539 Layer Erosion
Resistance Breakthrough Time, Secs 89 93 115 93 95 128 115 109
Calculated Erosion Rate, 141 188 255 215 232 224 220 237 secs/mil
Weight Loss in 30 seconds, g 0.00014 -- -- -- -- 0.00012 -- --
__________________________________________________________________________
Example 4I Example 4J Example
__________________________________________________________________________
4K Experiment No. 177 167 169 Substrate AM-350 Ti/6A1/4V IN-718
AM-350 Ti/6A1/4V IN-718 AM-350 Ti/6A1/4V IN-718 Coating Composition
W + W.sub.3 C W + W.sub.3 C W + W.sub.3 C Coating Hardness,
Kg/mm.sup.2 Tungsten Layer -- -- -- -- -- -- -- -- --
Tungsten/Tungsten Carbide Layer 1930 1997 1971 2508 2516 2567 -- --
-- Erosion Resistance Breakthrough Time, Secs 69 70 70 94 93 96 45
34 42 Calculated Erosion Rate, secs/mil 227 228 231 274 262 271 260
252 266 Weight Loss in 30 seconds, g -- -- -- -- -- -- 0.00014 --
--
__________________________________________________________________________
EXAMPLE 4C
The CVD run described in Example 4A was repeated to provide
tungsten followed by slightly thinner tungsten/tungsten carbide
coatings. The reaction conditions used in tungsten and
tungsten/tungsten carbide coating steps are summarized in Table
2.
The thicknesses of the tungsten and the tungsten/tungsten carbide
layers obtained on AM-350 and Ti/6Al/4V are summarized in Table 2.
The top coat of the coating consisted of a mixture of W and W.sub.3
C phases. The coating, once again, showed the presence of a network
of cracks. However, the crack density was considerably lower than
that observed in Examples 3, 4A and 4B. This suggested that the
tungsten interlayer was helpful in reducing the crack density. The
erosion resistance of the coating was considerably better then that
of the coatings obtained in Examples 3, 4A and 4B (see Table 3).
The extent of chipping observed during the erosion test also
reduced considerably.
This example showed that increasing the ratio of the thickness of
the tungsten layer to that of the tungsten/tungsten carbide layer
from .about.0.07 to 0.12 in Examples 4A and 4B to .about.0.2 to 0.3
in this example unexpectedly reduced cracks in the coating and
improved its erosion resistance.
EXAMPLE 4D
The CVD run described in Example 4A was once again repeated to
provide tungsten followed by tungsten/tungsten carbide coating. The
reaction conditions used for depositing tungsten and
tungsten/tungsten carbide coatings were selected in such a way to
provide a ratio of the thickness of the tungsten to the
tungsten/tungsten carbide layers of .about.0.5 (see Table 2).
The coating showed the presence of cracks, but the crack density
was greatly reduced. The coating, surprisingly, showed superior
erosion resistance compared to Examples 3 and 4A to 4C.
Furthermore, the composite coating obtained in this example showed
significantly lower weight loss in the erosion resistance test than
Example 3. The chipping of the coating observed during the erosion
test was reduced dramatically as well. This example, therefore,
clearly demonstrated the importance of the tungsten interlayer in
reducing cracks and improving erosion resistance of the composite
coating.
EXAMPLE 4E
In this example, reaction conditions for coating tungsten followed
by tungsten/tungsten carbide were selected in such a way to provide
a ratio of the thickness of the tungsten to the tungsten/tungsten
carbide layers of .about.0.68 (see Table 2).
The composite coating showed the presence of only a few fine cracks
(see FIG. 6). Etched cross section of the coating clearly showed
columnar tungsten interlayer followed by non-columnar W+W.sub.3 C
coating. The composite coating demonstrated superior erosion
performance as shown in Table 3.
This example clearly demonstrated the importance of the tungsten
interlayer in reducing cracks and improving erosion resistance of
the composite coating.
EXAMPLE 4F
In this example, reaction conditions for coating tungsten followed
by tungsten/tungsten carbide were selected in such a way to
slightly increase the thickness ratio (see Table 2).
Once again, the coating on AM-350 and Ti/6Al/4V showed presence of
a few cracks. Erosion resistance of the composite coating
summarized in Table 3 was better than Example 4E. This example also
demonstrated the importance of the tungsten interlayer in reducing
cracks and improving erosion resistance.
EXAMPLE 4G
The CVD run described in Example 4F was repeated using reaction
conditions summarized in Table 2 to provide slightly higher
tungsten to tungsten/tungsten carbide coating thickness ratio.
Once again, the coating showed presence of a few fine cracks.
Etched cross section of the coating clearly showed the presence of
columnar tungsten interlayer and non-columnar tungsten/tungsten
carbide top coat. The erosion resistance of the coating was very
similar to that observed in Example 4F.
EXAMPLE 4H
The CVD run described in Example 4G was repeated using reaction
conditions summarized in Table 2 to provide even higher tungsten to
tungsten/tungsten carbide coating thickness ratio.
The coating on AM-350, Ti/6Al/4V and IN-718 showed presence of a
few cracks. The erosion resistance of the coating was very similar
to that noted in Examples 4F and 4G. The weight loss during the
erosion test was considerably lower than that observed earlier.
This example, therefore, showed the importance of the tungsten
interlayer in reducing the cracks in the coating and in improving
the erosion resistance of the coating. It also showed that
increasing the thickness of the tungsten interlayer to .about.16
.mu.m without increasing that of the tungsten/tungsten carbide did
not help in further increasing the erosion resistance of the
coating.
EXAMPLE 4I
The CVD run described in Example 4H was repeated to provide even
higher tungsten to tungsten/tungsten carbide coating thickness
ratio. The higher ratio was obtained by reducing the thickness of
the tungsten/tungsten carbide layer.
The coating showed the presence of a few very fine cracks. The
erosion resistance of the composite coating was similar to that
noted in Example 4H (see Table 3). This example, once again, showed
the importance of the tungsten interlayer in reducing cracks and
improving erosion resistance of the composite coating. It also
showed that the breakthrough time was dependent on the thickness of
the tungsten/tungsten carbide layer; whereas, the erosion rate was
independent of it.
EXAMPLE 4J
The CVD run described in Example 4I was repeated to provide even
higher tungsten to tungsten/tungsten carbide coating thickness
ratio. The higher ratio was obtained by increasing the thickness of
the tungsten interlayer.
The coating, once again, showed presence of a few very fine cracks.
The erosion resistance of the composite coating was slightly higher
than that noted earlier (see Table 3). This example also showed the
importance of the tungsten interlayer.
EXAMPLE 4K
The CVD run described in Example 4J was repeated to provide even
higher tungsten to tungsten/tungsten carbide coating thickness
ratio.
The coating, unexpectedly, was found to be absolutely crack free,
as shown in the FIG. 7. The erosion resistance of the coating was
comparable to the data noted earlier. Weight loss during erosion
test was also comparable to the data noted earlier.
This example demonstrated that a crack-free coating could be
obtained by manipulating the ratio of the thickness of the tungsten
and the tungsten/tungsten carbide layers. This is an unexpected and
significant finding.
Discussion on Tungsten Followed by W+W.sub.3 C Coating
Because of extensive cracking, the W+W.sub.3 C coating without a W
interlayer provided very little erosion protection to the base
metal. In this case, erosion occurs preferentially at cracks which
causes chipping and flaking of large pieces of the coating. The
cracks are believed to occur during the deposition and/or cool-down
due to the build up in stresses within the coating. Surprisingly,
the cracks in the coating can be minimized or even eliminated by
providing a tungsten interlayer. It is believed that the function
of the tungsten interlayer is to accommodate stresses that build up
in the coating during the deposition and/or cool-down. The ability
to accommodate stress may be in part due to the columnar structure
of the tungsten layer, since the compliance of this layer is
probably very anisotropic. The amount of coating stress which can
be accommodated by the tungsten interlayer will depend on its
thickness. However, the physical dimensions over which a
significant amount of stress accommodation will occur will be
limited so that the stresses which are present in the outer layer
of a thick tungsten/tungsten carbide coating will be virtually
unaffected by the presence of the interlayer. Consequently, both
the thickness of the tungsten/tungsten carbide layer and the ratio
of the W to the W+W.sub.3 C thickness are important in obtaining a
crack-free composite coating.
The presence of a tungsten interlayer, in which the thickness of
the tungsten interlayer is at least 3 .mu.m and the ratio of W to
W+W.sub.3 C thickness is at least above 0.3, is necessary to
increase the erosion resistance of the composite coating. However,
the presence of a tungsten interlayer, in which the ratio of W to
W+W.sub.3 C thickness is at least 0.6, is necessary for the optimum
erosion and abrasion wear performance.
The tungsten interlayer also effects the performance of the
tungsten/tungsten carbide coating in a scratch test. In this test,
a diamond stylus is dragged across the surface of the sample at a
constantly increasing load. The load at which coating loss begins
to occur and the extent of coating loss can generally be correlated
to the performance of a coating in the erosive and abrasive wear
applications. The tungsten/tungsten carbide coating with no
tungsten interlayer as described in Example 3 showed extensive
coating loss in the 30-40 Newton load range in the scratch test, as
shown in the FIG. 8. When a tungsten interlayer is present as
described in Example 4E, the loss of the coating in the same load
range is significantly reduced, as demonstrated in the FIG. 9;
consequently, the presence of a tungsten interlayer is important
for erosive and abrasive wear applications.
Due to the preferential erosive and abrasive wear, which occurs at
cracks, a crack-free coating may be highly desirable under certain
wear conditions. This conclusion would be especially true in
situations where the substrate needs to be completely protected,
such as in corrosive-wear environments. A crack-free coating will
also be important when a smooth wear surface is required. This
crack-free coating was unexpectedly obtained by significantly
increasing the ratio of the thickness of the W to the W+W.sub.3 C
layer. Although Example 4K was produced by depositing a thin layer
of W+W.sub.3 C, in theory, a crack-free coating could also be
produced by depositing a very thick tungsten interlayer.
EXAMPLES FOR TUNGSTEN/TUNGSTEN CARBIDE (W+W.sub.2 C+W.sub.3 C)
COATING
EXAMPLE 5A
In this example, several AM-350 specimens were coated in a run. The
specimens were heated to a temperature of about 460.degree. C. in
the presence of flowing argon gas and at the reaction temperature a
gaseous mixture of 300 cc/min WF.sub.6, 3,000 cc/min of hydrogen
and 55 cc/min of DME was passed into the furnace over the
specimens. The total pressure was maintained at 40 Torr, as shown
in Table 4. The deposition was conducted for 20 minutes.
All the specimens were coated with a bright, smooth, adherent,
coherent and uniform coating. The coating thickness was .about.7
.mu.m (see Table 4). It consisted of a mixture of W, W.sub.2 C and
W.sub.3 C phases, which was considerably different than observed in
Example 3. It was free of columnar grains. The micro-structure of
the coating consisted of a layered structure. The coating had a
smooth surface finish. However, the surface of the coating was
cracked, as shown in the FIG. 10. The crack density was
surprisingly lower than that observed in Example 3. The coating had
a hardness of 2248 Vickers (Table 5). The coating showed poor
erosion resistance; the breakthrough time and erosion rate were 21
seconds and 76 seconds/mil. The erosion resistance, however, was
much higher than Example 3. The weight loss during erosion test was
0.00042 g in 30 seconds, which was very similar to that noted in
Example 3. Extensive chipping of the coating was observed during
the erosion test. Poor erosion resistance of the coating was
probably due to the presence of a network of cracks in the
coating.
EXAMPLE 5B
The CVD run described in Example 5A was repeated with Ti/6Al/4V
specimens using reaction conditions described in Table 4. The
specimens were coated with a bright, smooth, adherent, coherent and
uniform .about.9.5 .mu.m thick coating. It consisted of a mixture
of W, W.sub.2 C and W.sub.3 C phases. It was free of columnar
grains. It had a smooth surface finish. It consisted of a network
of cracks on the surface. It showed poor erosion resistance (see
Table 5), but the erosion resistance was
TABLE 4
__________________________________________________________________________
Example 5A Example 5B Example 6A Example 6B Example
__________________________________________________________________________
6C Experiment No. 51 140 70 72 77 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V AM-350 Ti/6A1/4V AM-350 Ti/6A1/4V Temperature, .degree.C.
460 460 460 465 465 Pressure, Torr 40 40 40 40 40 W Coating
Conditions H.sub.2, SCCM -- -- 3,000 3,000 3,000 Ar, SCCM -- --
4,500 4,500 4,500 WF.sub.6, SCCM -- -- 300 300 300 Time, Min. -- --
3 4.5 10 Tungsten Carbide Conditions H.sub.2, SCCM 3,000 3,000
3,000 3,000 3,250 Ar, SCCM -- 1,800 -- -- -- WF.sub.6, SCCM 300 300
300 300 300 DME, SCCM 55 55 55 55 60 Time, Min. 20 35 20 20 45
Coating Thickness, .mu.m Tungsten -- -- 3.0 3.4 5.0 5.0 8.0 8.0
Tungsten/Tungsten Carbide 7.0 9.5 8.4 9.0 13.0 9.5 20.0 15.0
Tungsten Coating Thickness Tungsten/Tungsten Carbide -- -- 0.36
0.38 0.38 0.52 0.40 0.53 Thickness Surface Topography Smooth, Many
Smooth, Many Smooth, A Few Inter- Smooth, A Few Fine Smooth, A
Interconnected Interconnected connected Cracks Cracks Few Cracks
Cracks Cracks
__________________________________________________________________________
Example 6D Example 6E Example 6F Example
__________________________________________________________________________
6G Experiment No. 53 103 82 111 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V AM-350 Ti/6A1/4V AM-350 Ti/6A1/4V Temperature, .degree.C.
460 460 460 465 Pressure, Torr 40 40 40 40 W Coating Conditions
H.sub.2, SCCM 3,000 3,000 3,000 3,000 Ar, SCCM 4,000 4,500 4,500
4,500 WF.sub.6, SCCM 300 300 300 300 Time, Min. 7 15 10 15 Tungsten
Carbide Conditions H.sub.2, SCCM 3,000 3,000 3,500 3,000 Ar, SCCM
-- 300 -- 300 WF.sub.6, SCCM 300 300 350 300 DME, SCCM 55 60 65 50
Time, Min. 20 40 30 30 Coating Thickness, .mu.m Tungsten 6.0 6.0
9.2 7.7 9.0 8.6 9.6 8.8 Tungsten/Tungsten Carbide 9.5 9.5 12.3 12.3
16.0 11.5 11.5 12.0 Tungsten Coating Thickness Tungsten/Tungsten
Carbide 0.63 0.63 0.74 0.63 0.56 0.75 0.83 0.73 Thickness Surface
Topography Smooth, Smooth, Smooth, A Few Fine Smooth, A Few Fine
Smooth, A Few Fine No A Few Cracks Cracks Cracks Cracks Cracks
__________________________________________________________________________
Example 6H Example 6I Example 6J Example
__________________________________________________________________________
6K Experiment No. 102 52 56 76 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V AM-350 Ti/6A1/4V AM-350 Ti/6A1/4V Temperature, .degree.C.
460 460 460 465 Pressure, Torr 40 40 40 40 W Coating Conditions
H.sub.2, SCCM 3,000 3,000 3,000 3,000 Ar, SCCM 4,500 4,000 4,500
4,000 WF.sub.6, SCCM 300 300 300 300 Time, Min. 15 10 10 30
Tungsten Carbide Conditions H.sub.2, SCCM 3,000 3,000 3,000 3,000
Ar, SCCM 300 -- -- -- WF.sub.6, SCCM 300 300 300 300 DME, SCCM 60
55 55 55 Time, Min. 30 20 15 45 Coating Thickness, .mu.m Tungsten
10.0 8.1 8.0 7.5 7.1 7.5 19.0 16.0 Tungsten/Tungsten Carbide 8.0
8.0 10.0 7.5 6.2 6.0 11.0 9.0 Tungsten Coating Thickness
Tungsten/Tungsten Carbide 1.25 1.01 0.80 1.00 1.14 1.25 1.75 1.78
Thickness Surface Topography Smooth, No Cracks Smooth, No Cracks
Smooth, No Cracks Smooth, No
__________________________________________________________________________
Cracks
TABLE 5
__________________________________________________________________________
Example 5A Example 5B Example 6A Example
__________________________________________________________________________
6B Experiment No. 51 140 70 72 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V AM-350 Ti/6A1/4V Coating Composition W + W.sub.2 C +
W.sub.3 C W + W.sub.2 C + W.sub.3 C W + W.sub.2 C + W.sub.3 C W +
W.sub.2 C + W.sub.3 C Coating Hardness, Kg/mm.sup.2 Tungsten Layer
-- -- -- -- -- -- Tungsten/Tungsten Carbide Layer 2248 .+-. 70 2191
2078 .+-. 49 -- 2253 ---. 73 Erosion Resistance Breakthrough Time,
secs 21 44 104 131 160 147 Calculated Erosion Rate, 76 117 314 370
313 394 secs/mil Weight Loss in 0.00042 0.0028 0.00012 0.00015 --
-- 30 seconds, g
__________________________________________________________________________
Example 6C Example 6D Example
__________________________________________________________________________
6E Experiment No. 77 53 103 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V AM-350 Ti/6A1/4V Coating Composition W + W.sub.2 C +
W.sub.3 C W + W.sub.2 C + W.sub.3 C W + W.sub.2 C + W.sub.3 C
Coating Hardness, Kg/mm.sup.2 Tungsten Layer -- -- -- -- -- --
Tungsten/Tungsten Carbide Layer 2102 .+-. 101 -- 2359 .+-. 114 2215
.+-. 194 2224 2469 .+-. 53 Erosion Resistance Breakthrough Time,
secs 218 201 119 -- 158 137 Calculated Erosion Rate, 278 340 318 --
325 283 secs/mil Weight Loss in -- -- -- -- -- -- 30 seconds, g
__________________________________________________________________________
Example 6F Example 6G Example
__________________________________________________________________________
6H Experiment No. 82 111 102 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V AM-350 Ti/6A1/4V Coating Composition W + W.sub.2 C +
W.sub.3 C W + W.sub.2 C + W.sub.3 C W + W.sub.2 C + W.sub.3 C
Coating Hardness, Kg/mm.sup.2 Tungsten Layer -- -- -- -- -- --
Tungsten/Tungsten Carbide Layer 2220 .+-. 100 -- 3097 .+-. 210 3156
.+-. 76 2167 2324 .+-. 50 Erosion Resistance Breakthrough Time,
secs 209 128 146 123 67 66 Calculated Erosion Rate, 332 284 322 260
212 208 secs/mil Weight Loss in -- -- -- -- -- -- 30 seconds, g
__________________________________________________________________________
Example 6I Example 6J Example
__________________________________________________________________________
6K Experiment No. 52 56 76 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V AM-350 Ti/6A1/4V Coating Composition W + W.sub.2 C +
W.sub.3 C W + W.sub.2 C + W.sub.3 C W + W.sub.2 C + W.sub.3 C
Coating Hardness, Kg/mm.sup.2 Tungsten Layer -- -- -- -- -- --
Tungsten/Tungsten Carbide Layer 2091 .+-. 101 2155 .+-. 20 2078
.+-. 66 -- -- 2091 .+-. 98 Erosion Resistance Breakthrough Time,
secs 125 73 58 91 158 150 Calculated Erosion Rate, 317 248 237 383
364 423 secs/mil Weight Loss in 0.00012 0.00014 -- -- -- -- 30
seconds, g
__________________________________________________________________________
TABLE 6
__________________________________________________________________________
Example 7A Example 7B Example 7C
__________________________________________________________________________
Experiment No. 69 121 115 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V IN-718 AM-350 Ti/6A1/4V IN-718 Temperature, .degree.C.
460 440 460 Pressure, Torr 40 40 40 W Coating Conditions H.sub.2,
SCCM 3,000 2,500 3,000 Ar, SCCM 4,500 5,500 4,500 WF.sub.6, SCCM
300 250 300 Time, Min. 15 15 10 Tungsten/Tungsten Carbide
Conditions H.sub.2, SCCM 3,000 3,000 3,000 Ar, SCCM -- 1,800 300
WF.sub.6, SCCM 300 300 300 DME, SCCM 85 90 90 Time, Min. 70 115 40
Coating Thickness, .mu.m Tungsten 8.0 10.0 6.8 6.0 6.3 4.8 4.6 5.0
Tungsten/Tungsten Carbide 23.0 23.0 17.0 16.0 15.0 11.0 10.0 10.0
Tungsten Coating Thickness Tungsten/Tungsten Carbide 0.35 0.43 0.40
0.37 0.42 0.44 0.46 0.50 Thickness Surface Topography Smooth, A few
fine Smooth, a few Fine Smooth, Smooth, Smooth, Interconnected
Cracks No Cracks Fine Noacks Cracks Cracks
__________________________________________________________________________
Example 7D Example 7E Example 7F Example
__________________________________________________________________________
7G Experiment No. 95 120 119 91 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V IN-718 AM-350 Ti/6A1/4V AM-350 Ti/6A1/4V Temperature,
.degree.C. 460 445 450 460 Pressure, Torr 40 40 40 40 W Coating
Conditions H.sub.2, SCCM 3,000 2,500 2,500 3,000 Ar, SCCM 4,500
5,500 5,500 4,500 WF.sub.6, SCCM 300 250 250 300 Time, Min. 10 15
15 10 Tungsten/Tungsten Carbide Conditions H.sub.2, SCCM 3,000
3,000 3,000 3,000 Ar, SCCM 300 1,800 1,500 300 WF.sub.6, SCCM 300
300 300 300 DME, SCCM 90 90 90 90 Time, Min. 50 95 80 40 Coating
Thickness, .mu.m Tungsten 6.6 4.4 6.8 6.2 6.4 6.8 6.8 5.6 5.0
Tungsten/Tungsten Carbide 14.8 12.0 13.0 12.7 10.6 12.4 12.8 10.0
10.0 Tungsten Coating Thickness Tungsten/Tungsten Carbide 0.44 0.37
0.52 0.49 0.60 0.55 0.53 0.56 0.50 Thickness Surface Topography
Smooth, Fine Cracks Smooth, No Cracks Smooth, No Cracks Smooth, No
__________________________________________________________________________
Cracks Example 7H Example 7I Example
__________________________________________________________________________
7J Experiment No. 97 122 68 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V IN-718 AM-350 Ti/6A1/4V Temperature, .degree.C. 460 460
460 Pressure, Torr 40 40 40 W Coating Conditions H.sub.2, SCCM
3,000 2,500 3,000 Ar, SCCM 4,500 5,500 4,500 WF.sub.6 , SCCM 300
250 300 Time, Min. 15 25 30 Tungsten/Tungsten Carbide Conditions
H.sub.2, SCCM 3,000 3,000 3,000 Ar, SCCM 300 1,500 -- WF.sub.6,
SCCM 300 300 300 DME, SCCM 90 90 85 Time, Min. 50 85 70 Coating
Thickness, .mu.m Tungsten 9.0 7.0 12.4 12.3 12.1 25.0 23.0
Tungsten/Tungsten Carbide 15.0 11.5 19.1 18.3 17.5 27.0 26.0
Tungsten Coating Thickness Tungsten/Tungsten Carbide 0.60 0.61 0.65
0.67 0.69 0.93 0.88 Thickness Surface Topography Smooth, Smooth,
Smooth, Cracks Smooth, Cracks Cracks No Cracks
__________________________________________________________________________
Example 7K Example 7L Example 7M Example
__________________________________________________________________________
7N Experiment No. 123 100 101 63 Substrate AM-350 Ti/6A1/4V IN-718
AM-350 Ti/6A1/4V AM-350 Ti/6A1/4V AM-350 Ti/6A1/4V Temperature,
.degree.C. 460 460 460 460 Pressure, Torr 40 40 40 40 W Coating
Conditions H.sub.2, SCCM 2,500 3,000 3,000 3,000 Ar, SCCM 5,500
4,500 4,500 4,500 WF.sub.6, SCCM 250 300 300 300 Time, Min. 18 15
15 10 Tungsten/Tungsten Carbide Conditions H.sub.2, SCCM 3,000
3,000 3,000 3,000 Ar, SCCM 1,800 300 300 300 WF.sub.6, SCCM 300 300
300 300 DME, SCCM 90 80 70 85 Time, Min. 65 40 40 20 Coating
Thickness, .mu.m Tungsten 8.8 9.0 9.2 12.0 10.4 11.2 9.2 7.5 6.7
Tungsten/Tungsten Carbide 12.7 13.1 12.3 11.6 10.4 10.0 9.2 7.7 6.5
Tungsten Coating Thickness Tungsten/Tungsten Carbide 0.69 0.68 0.75
1.03 1.00 1.12 1.00 0.97 1.03 Thickness Surface Topography Smooth,
No Cracks Smooth, No Cracks Smooth, No Cracks Smooth, No
__________________________________________________________________________
Cracks Example 7O Example
__________________________________________________________________________
7P Experiment No. 65 90 Substrate AM-350 Ti/6A1/4V AM-350 Ti/6A1/4V
Temperature, .degree.C. 460 460 Pressure, Torr 40 40 W Coating
Conditions H.sub.2, SCCM 3,000 3,000 Ar, SCCM 4,500 4,500 WF.sub.6,
SCCM 300 300 Time, Min. 5 10 Tungsten/Tungsten Carbide Conditions
H.sub.2, SCCM 3,000 3,000 Ar, SCCM 300 300 WF.sub.6, SCCM 300 300
DME, SCCM 90 90 Time, Min. 20 30 Coating Thickness, .mu.m Tungsten
4.0 4.0 5.6 5.2 Tungsten/Tungsten Carbide 6.2 5.9 7.6 7.4 Tungsten
Coating Thickness Tungsten/Tungsten Carbide 0.64 0.68 0.74 0.70
Thickness Surface Topography Smooth, No Cracks Smooth, No
__________________________________________________________________________
Cracks
TABLE 7
__________________________________________________________________________
Example 7A Example 7B Example 7C
__________________________________________________________________________
Experiment No. 69 121 115 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V IN-718 AM-350 Ti/6A1/4V IN-718 Coating Composition W +
W.sub.2 C W + W.sub.2 C W + W.sub.2 C Coating Hardness, Kg/mm.sup.2
Tungsten Layer -- -- -- -- -- -- -- -- Tungsten/Tungsten 3060 .+-.
230 2792 .+-. 60 2470 .+-. 53 2432 .+-.53 2588 .+-. 57 2850 .+-. 66
2947 ---. 71 Carbide Layer Erosion Resistance Breakthrough Time,
secs -- 304 201 214 183 135 126 116 Calculated Erosion Rate, -- 336
300 339 310 312 319 295 secs/mil Weight Loss in -- -- 0.00018
0.00020 0.00024 -- -- -- 30 seconds, g
__________________________________________________________________________
Example 7D Example 7E Example
__________________________________________________________________________
7F Experiment No. 95 120 119 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V IN-718 AM-350 Ti/6A1/4V Coating Composition W + W.sub.2 C
W + W.sub.2 C W + W.sub.2 C Coating Hardness, Kg/mm.sup.2 Tungsten
Layer -- -- -- -- -- -- -- Tungsten/Tungsten 2991 .+-. 63 3106 .+-.
148 2628 .+-. 15 2507 .+-. 20 2402 .+-. 160 2469 .+-. 53 2395 .+-.
15 Carbide Layer Erosion Resistance Breakthrough Time, secs 203 149
132 135 112 165 154 Calculated Erosion Rate, 349 311 258 270 284
325 306 secs/mil Weight Loss in -- -- -- -- -- -- -- 30 seconds, g
__________________________________________________________________________
Example 7G Example 7H Example 7I
__________________________________________________________________________
Experiment No. 91 97 122 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V AM-350 Ti/6A1/4V IN-718 Coating Composition W + W.sub.2 C
W + W.sub.2 C W + W.sub.2 C Coating Hardness, Kg/mm.sup.2 Tungsten
Layer -- -- -- -- -- -- -- Tungsten/Tungsten 3089 .+-. 89 2767 .+-.
188 2908 .+-. 213 2758 .+-. 15 2472 .+-. 110 2507 .+-. 15 2432 .+-.
53 Carbide Layer Erosion Resistance Breakthrough Time, secs 124 113
182 150 259 234 248 Calculated Erosion Rate, 315 288 308 331 345
325 361 secs/mil Weight Loss in -- -- -- -- -- -- -- 30 seconds, g
__________________________________________________________________________
Example 7J Example 7K Example
__________________________________________________________________________
7L Experiment No. 68 123 100 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V IN-718 AM-350 Ti/6A1/4V Coating Composition W + W.sub.2 C
W + W.sub.2 C W + W.sub.2 C Coating Hardness, Kg/mm.sup.2 Tungsten
Layer -- -- Tungsten/Tungsten 2758 .+-. 31 2689 .+-. 159 2398 .+-.
109 2324 .+-. 50 2511 .+-. 116 2758 .+-. 15 2758 .+-. 15 Carbide
Layer Erosion Resistance Breakthrough Time, secs 356 325 162 172
143 144 120 Calculated Erosion Rate, 335 318 325 328 295 316 293
secs/mil Weight Loss in -- -- 0.00016 0.00018 0.00014 -- -- 30
seconds, g
__________________________________________________________________________
Example 7M Example 7N Example 7O Example
__________________________________________________________________________
7P Experiment No. 101 62 65 90 Substrate AM-350 Ti/6A1/4V AM-350
Ti/6A1/4V AM-350 Ti/6A1/4V AM-350 Ti/6A1/4V Coating Composition W +
W.sub.2 C W + W.sub.2 C W + W.sub.2 C W + W.sub.2 C Coating
Hardness, Kg/mm.sup.2 Tungsten Layer -- -- -- -- -- -- -- --
Tungsten/Tungsten 2434 .+-. 102 2510 .+-. 95 2746 .+-. 51 -- 2758
.+-. 50 -- 2660 2716 .+-. 163 Carbide Layer Erosion Resistance
Breakthrough Time, secs 125 125 -- 65 54 65 92 73 Calculated
Erosion Rate, 317 345 -- 254 221 280 307 250 secs/mil Weight Loss
in -- -- -- -- -- -- -- -- 30 seconds, g
__________________________________________________________________________
This example showed that the erosion resistance of W+W.sub.2 C
coating with a W interlayer was much better than that of W+W.sub.3
C and W+W.sub.2 C+W.sub.3 C coatings with a W interlayer. This is
an unexpected and significant finding.
EXAMPLE 7B
In this example, several AM-350, Ti/6Al/4V and IN-718 specimens
were coated with W followed by W+W.sub.2 C coating using conditions
summarized in Table 6. All the specimens were coated with .about.6
.mu.m and .about.15 .mu.m W and W+W.sub.2 C layers, respectively.
The coating consisted of columnar W coating and non-columnar
W+W.sub.2 C coating at the top of W. The thickness ratio was
.about.0.4. The coating was smooth. However, a few fine cracks were
noted in the coating, as shown in the FIG. 12. The erosion
resistance of the coating shown in Table 7 was similar to that
noted in Example 7A. The weight loss during erosion test was
determined and presented in Table 7. The weight loss was slightly
higher in this example than that observed with W+W.sub.2 C+W.sub.3
C coating with W interlayer. This could be related to the fact the
cracks in W+W.sub.2 C coatings were slightly wider than those noted
in W+W.sub.2 C+W.sub.3 C coating.
Examples 7A and 7B clearly showed that a tungsten to
tungsten/tungsten carbide thickness ratio of .about.0.35 is
sufficient enough to give a maximum erosion resistance.
EXAMPLE 7C
The CVD run described in Example 7B was repeated to provide
slightly higher thickness ratio. The thickness of W+W.sub.2 C layer
varied from 10 .mu.m to 11 .mu.m on various substrates, and the
ratio varied from 0.44 to 0.50. The coating on AM-350 and IN-718
was crack-free; whereas, a few fine cracks were observed on
Ti/6Al/4V. This indicated that crack-free coating on AM-350 and
IN-718 can be obtained by providing .about.5 .mu.m thick W and
.about.10 to 11 .mu.m thick W+W.sub.2 C layers. These thickness
values, however, were not good enough to give crack-free coating on
Ti/6Al/4V. The erosion resistence of the coating was very good as
shown in Table 7.
EXAMPLE 7D
The CVD run described in Example 7C was repeated to provide
slightly thicker W+W.sub.2 C layer while maintaining thickness
ratio. Surprisingly, the coating on AM-350 and Ti/6Al/4V cracked.
This information suggested that the thickness of W+W.sub.2 C layer
was important to prevent cracks in the coating. The erosion
resistance of the coating was good, as shown in Table 7.
EXAMPLE 7E
The CVD run described in Example 7D was repeated to provide
slightly thinner W+W.sub.2 C layer and slightly higher thickness
ratio. The coating on AM-350, Ti/6Al/4V and IN-718 was absolutely
crack-free (see FIG. 13), certifying the importance of the
thickness of W+W.sub.2 C. The unetched and etched cross-sectional
views of the composite coatings are shown in the FIGS. 3 and 4. The
etched cross-section presented in the FIG. 4 showed columnar growth
of the tungsten interlayer and the non-columnar growth of the
tungsten/tungsten carbide layer. The etched cross-section of the
coating also demonstrated the absence of any cracks in the coating.
The erosion resistance of the coating was good, as shown in Table
7.
EXAMPLES 7F AND 7G
The CVD run described in Example 7E was repeated in these examples
to verify the concept of crack-free coating. The coatings obtained
in these runs were absolutely crack-free. They also had good
erosion resistance, as shown in Table 7.
EXAMPLE 7H
In this example, the thickness of W and W+W.sub.2 C layers was
slightly increased on AM-350 stainless steel to determine its
effect on cracks. The thickness ratio obtained was .about.0.6. The
coating on AM-350 cracked and that on Ti/6Al/4V did not. The
coating on AM-350 cracked probably because of the thicker W+W.sub.2
C layer. This information further confirmed the statement made
earlier that the thickness of W+W.sub.2 C layer played an important
role in obtaining crack-free coating. Erosion resistance of the
coating was good, as shown in Table 7.
EXAMPLE 7I
To further demonstrate the effect of W+W.sub.2 C coating thickness
on cracks, a CVD run was carried out to obtain a thicker W+W.sub.2
C coating and a higher thickness ratio. Despite higher thickness
ratio the coating on all the specimens cracked. This example,
therefore, confirmed the importance of the thickness of W+W.sub.2 C
layer for preventing cracks in the coating. The erosion resistance
of the coating was good, as expected.
EXAMPLE 7J
To further demonstrate the effect of W+W.sub.2 C coating thickness
on cracks, one more CVD run was conducted to obtain thicker
W+W.sub.2 C coating and higher thickness ratio. Once again, coating
cracked. The erosion resistance of the coating, however, was still
good.
EXAMPLES 7K TO 7P
Several CVD runs were conducted to vary thicknesses of W and
W+W.sub.2 C layers and thickness ratios. These experiments were
conducted to effectively map out the crack-free and cracked coating
region. The data summarized in Table 6 showed that as long as the
thickness of W+W.sub.2 C layer was maintained below .about.13.5
.mu.m and thickness ratio maintained above 0.6 the coating obtained
was absolutely crack-free. The coating in all the cases
demonstrated good erosion resistance, as shown in Table 7.
Discussion on Tungsten Followed by W+W.sub.2 C Coating
The erosion resistance of the W+W.sub.2 C coating is either
equivalent to or superior to the erosion resistance of W+W.sub.2
C+W.sub.3 C. The erosion resistance of W+W.sub.2 C is independent
of the thickness of the tungsten interlayer when the ratio of the W
to the W+W.sub.2 C thickness is at least 0.35.
Careful control of both the thickness of the tungsten/tungsten
carbide coating and the ratio of the thicknesses of W to W+W.sub.2
C is required to obtain a crack-free coating. For both AM-350 and
Ti/6Al/4V, the maximum thickness for producing a crack-free coating
is about 14 .mu.m. The critical thickness ratio for a crack-free
coating is about 0.4 and 0.5 for AM-350 and Ti/6Al/4V,
respectively.
EXAMPLE 8
The wear performance of the uncoated stainless steel 1" diameter
SS-422 disc was determined using a ball-on-disc test in the
presence and absence of a lubricant. The non-lubricated wear test
was conducted in dry air (1% relative humidity) and saturated air
(99% relative humidity). The lubricated wear test was conducted in
the presence of a cutting fluid consisting of an emulsion of 20%
mineral oil in water. The ball used in the test was made of 52-100
chrome steel. The ball-on-disc wear test was performed using a load
of 5 Newton, ambient temperature, stationary ball on rotating disc
at a speed of 10 cm/sec., and for approximately 0.3 kilometer. The
wear performance was determined by measuring the combined
volumetric material loss of the ball and disc. The wear rate was
very high in the dry air (1% relative humidity), as shown in the
Table 8. The wear rate in the saturated air (99% relative humidity)
and cutting fluid, on the other hand, was considerably lower, with
the rate being lowest in the presence of the lubricant.
EXAMPLE 9
In this example, a two-step coating process was used. Several
SS-422 discs were heated to a temperature of bout 460.degree. C. in
the presence of flowing argon and at the reaction temperature a
gaseous mixture of 300 cc/min WF.sub.6, 3,000 cc/min hydrogen, and
4,500 cc/min of argon was passed into the furnace over the
specimens for 15 minutes to coat them with tungsten. After coating
the specimens with tungsten for 15 minutes, a gaseous mixture of
300 cc/min WF.sub.6, 3,000 cc/min hydrogen, 300 cc/min argon and 40
cc/min of DME was passed into the furnace for 30 minutes to provide
tungsten/tungsten carbide coating. A total pressure was maintained
at 40 Torr during the tungsten as well as tungsten/tungsten carbide
coating steps.
The SS-422 discs were coated with 10.4 .mu.m thick tungsten
followed by 12.4 .mu.m thick tungsten/tungsten carbide. The
tungsten/tungsten carbide top coat consisted of a mixture of W and
W.sub.3 C phases. The hardness of the top coat was approximately
2450 Vickers. The coating was smooth and had a few very fine
interconnected cracks.
The wear performance of the coated SS-422 disc was determined using
ball-on-disc test described in the Example 8 in the presence of 1%
and 99% relative humidity and cutting fluid as a lubricant. The
wear rate data summarized in the Table 8 showed dramatically lower
wear rate in the presence of dry air (1% relative humidity)
compared to the uncoated disc. The wear rate in the presence of 99%
relative humidity and cutting fluid was also lower than the
uncoated disc.
This example, therefore, shows that a composite coating is very
effective in reducing the total wear rate in the presence and
absence of a lubricant.
EXAMPLE 10
In this example, a two-step coating process was used again. Several
SS-422 discs were heated to a temperature of about 460.degree. C.
in the presence of flowing argon and at the reaction temperature a
gaseous mixture of 300 cc/min WF.sub.6, 3,000 cc/min hydrogen, and
4,500 cc/min of argon was passed into the furnace over the
specimens for 15 minutes to coat them with tungsten. After coating
the specimens with tungsten for 15 minutes, a gaseous mixture of
300 cc/min WF.sub.6, 3,000 cc/min hydrogen, 300 cc/min argon and 60
cc/min of DME was passed into the furnace for 40 minutes to provide
tungsten/tungsten carbide coating. A total pressure was maintained
at 40 Torr during the tungsten as well as tungsten/tungsten carbide
coating steps.
The SS-422 discs were coated with 9.7 .mu.m thick tungsten followed
by 14.0 .mu.m thick tungsten/tungsten carbide. The
tungsten/tungsten carbide top coat consisted of a mixture of W,
W.sub.2 C and W.sub.3 C phases. It had a hardness of approximately
2250 Vickers. The coating was smooth and had a few extremely fine
and long cracks.
TABLE 8
__________________________________________________________________________
Composition of the Tungsten/Tungsten Total Wear Rate, 10.sup.-15
m.sup.2 /N Example No. Carbide Top Coat 1% Humidity 99% Humidity
Cutting Fluid
__________________________________________________________________________
Example 8 -- 327 8.1 2.9 Example 9 W + W.sub.3 C 1.9 7.7 1.0
Example 10 W + W.sub.2 C + W.sub.3 C 1.0 4.7 0.9 Example 11 W +
W.sub.2 C 1.40 3.6 1.0
__________________________________________________________________________
The wear performance of the coated SS-422 disc was determined using
ball-on-disc test described in the Example 8 in the presence of 1%
and 99% relative humidity and cutting fluid as a lubricant. The
wear rate data summarized in the Table 8 showed lower wear rate in
the presence of 1% and 99% relative humidity and cutting fluid
compared to the uncoated disc and disc coated with tungsten
followed by W+W.sub.3 C coating.
This example shows that a composite coating is very effective in
improving wear performance of the SS-422 disc. It also shows that
the wear performance of the composite coating can be improved by
adjusting the composition of the tungsten/tungsten carbide top
coat.
EXAMPLE 11
In this example, a two-step coating process was used again. Several
SS-422 discs were heated to a temperature of about 460.degree. C.
in the presence of flowing argon and at the reaction temperature a
gaseous mixture of 300 cc/min WF.sub.6, 3,000 cc/min hydrogen, and
4,500 cc/min of argon was passed into the furnace over the
specimens for 15 minutes to coat them with tungsten. After coating
the specimens with tungsten for 15 minutes, a gaseous mixture of
300 cc/min WF.sub.6, 3,000 cc/min hydrogen, 300 cc/min argon and 80
cc/min of DME was passed into the furnace for 40 minutes to provide
tungsten/tungsten carbide coating. A total pressure was maintained
at 40 Torr during the tungsten as well as tungsten/tungsten carbide
coating steps.
The SS-422 discs were coated with 10 .mu.m thick tungsten followed
by 13.0 .mu.m thick tungsten/tungsten carbide. The
tungsten/tungsten carbide top coat consisted of a mixture of W and
W.sub.2 C phases. It had a hardness of approximately 2750 Vickers.
The coating was smooth and crack-free.
The wear performance of the coated SS-422 disc was determined using
ball-on-disc test described in Example 8 in the presence of 1% and
99% relative humidity and cutting fluid as a lubricant. The wear
rate data summarized in Table 8 showed the wear rate was similar to
that noted with the other composite coatings (i.e., W+W.sub.3 C and
W+W.sub.2 C+W.sub.3 C coatings) in the presence of dry air (1%
relative humidity) and cutting fluid. The wear rate, however, was
lower compared to other composite coatings in the presence of 99%
relative humidity.
This example shows that a composite coating is very effective in
improving wear performance of the SS-422 disc.
General Discussion
The wear data presented in Examples 8 to 11 clearly demonstrate
that a composite tungsten followed by tungsten/tungsten carbide
coating can be used to significantly reduce the abrasive wear rate
and concomitantly increase the life of the stainless steel material
in the dry, humid and lubricated environments. The data presented
in Examples 1 to 4 show that the composite coating is very
effective in reducing the erosive wear rate of the ferrous and
non-ferrous alloys. Additionally, Examples 1 to 4 show that an
interlayer of tungsten is required to improve the performance of
the tungsten/tungsten carbide coating. This is an unexpected
finding.
The relationship between the ratio of the thickness of the tungsten
interlayer to the thickness of the tungsten-carbon alloy
(tungsten/tungsten carbide coating) is elaborated further in the
FIG. 14. It shows that for W+W.sub.3 C top coat the erosion
resistance measured as secs/mil increases with increasing the
thickness ratio. A thickness ratio greater than 0.3 is required to
significantly increase the erosion resistance of the W+W.sub.3 C
coating system. Furthermore, a thickness ratio of about 0.6 is
required to obtain optimum erosion resistance of the W+W.sub.3 C
coating system. FIG. 14 also shows that a thickness ratio of
greater than 0.3 is required for significantly increasing the
erosion resistance of the W+W.sub.2 C+W.sub.3 C coating system. It
also shows that a thickness ratio of about 0.35 yields optimum
erosion performance both for the W+W.sub.2 C+W.sub.3 C and
W+W.sub.2 C coating systems. This thickness ratio to obtain optimum
erosion resistance is considerably lower for the W+W.sub.2
C+W.sub.3 C and W+W.sub.2 C coating systems than the W+W.sub.3 C
coating system.
The relationship between the thickness of the tungsten-carbon alloy
and the ratio of the thickness of the tungsten interlayer to the
thickness of the tungsten-carbon alloy (tungsten/tungsten carbide
coating) is presented in FIG. 15. It shows a very narrow region for
obtaining a crack-free W+W.sub.3 C coating system. A thin W+W.sub.3
C layer is required to achieve a crack-free coating. Compared to
the W+W.sub.3 C coating system, the W+W.sub.2 C+W.sub.3 C and
W+W.sub.2 C coating systems provide a wider crack-free coating
region.
It is worth noting that a thicker crack-free tungsten/tungsten
carbide coating can be obtained for W+W.sub.2 C top coat than is
possible for W+W.sub.2 C+W.sub.3 C or for W+W.sub.3 C. In addition,
the thickness of the tungsten interlayer which is required to
achieve a crack-free coating is significantly lower for W+W.sub.2 C
than for W+W.sub.2 C+W.sub.3 C or W+W.sub.3 C. By reducing the
thickness of the W interlayer, the overall thickness of the
composite coating which is required to provide a specified erosive
wear life or abrasive wear life can be minimized by going
progressively from W+W.sub.3 C to W+W.sub.2 C+W.sub.3 C and to
W+W.sub.2 C composite coating system.
One particularly important use of the composite coating system
according to the present invention is to provide highly erosive and
abrasive wear resistant coatings on ferrous, non-ferrous and
titanium alloy compressor blades for gas turbines and jet
engines.
* * * * *